Hydraulic valves and systems are often used to transmit and control power through a fluid under pressure within an enclosed circuit. Power is usually controlled by maintaining an appropriate pressure and flow in a system or a part or component of the system. Load sense features are used in hydraulic systems to send information about actual load value to a control element. Usually the load sensing mechanism is simply a hydraulic line connecting a line before an actuator or a line with reference pressure level with a control device, like a pressure compensator. The latter is often used in flow control systems for achieving a high quality flow control.
A desirable flow rate may be constant or variable, and an appropriate flow control element may have a fixed or an adjustable opening for flow passage. In any case, flow through the control element depends not only on the size of the opening but also on the pressure drop across the opening. Special pressure compensators can be used to provide precise flow control regardless of the load or supply pressure. A pressure compensator is intended to provide a constant, relatively small pressure drop across a control element, for example, a fixed or adjustable orifice.
The main feature of a pressure compensator is a spool moveably disposed within a cage or a body. One side of the spool is connected to an input line of a control element; the opposite side is connected to an output line of the same control element. Another part of the compensator is a spring for pushing the spool in the direction of the side connected to the input line of the control element. In an equilibrium spool position, a force created by the input pressure acting on one side of the spool is equal to a force created by the outlet pressure acting on the other side of the spool in combination with a spring force. Any imbalance of the forces acting on the spool causes spool movement, which, in turn, changes the spool opening and adjusts the flow across the control element. Thus, the pressure differential across the control element, which is the spring force divided by the spool cross-sectional area, remains essentially the same regardless of the load or supply pressure, thereby making the flow through a control element essentially independent of load or supply pressure and being defined only by the opening of the control element.
System stability can be a desirable feature of a hydraulic system. Inasmuch as the opposite sides of a pressure compensator spool are connected to inlet and outlet lines of a control element, at least one of these lines is connected to a load and can be considered a load sense line. Flow in a load sense line is generally low as it is defined mainly by spool-body leakage and by spool displacement. One way to improve system stability is to provide a restrictive orifice in the load sense line for dampening spool movement. Though such an orifice improves stability, in some cases it makes the system sluggish in that the flow restriction causes an increased response time.